The invention relates to methods of directed self-assembly to form layered structures and more specifically, to the use of non-crosslinking photoresists developed using a non-alkaline solvent for directed polymer self-assembly.
Patterning features with smaller critical dimensions at tighter pitches allows denser circuitry to be created, and therefore can reduce overall production cost and improve device performance. Directed polymer self-assembly (DSA) is a potential candidate to extend current lithography by enhancing the spatial resolution and/or controlling the critical dimension variation of a predefined pattern on a substrate. There are two methods for implementing DSA: graphoepitaxy and chemical epitaxy. In a graphoepitaxial DSA technique, self-assembly (SA) of a material (e.g., a polymer) is guided by topographical features and their surface properties in lithographically pre-patterned substrates. In a chemical epitaxy DSA technique, self-assembly of a material is guided by lithographically defined chemical pre-patterns on a substrate surface. Two major challenges impeding integration of DSA into standard lithographic processes are non-compatibility of a patterned photoresist to the casting solvent used with SA materials and inability of the underlying substrate to support a specific SA morphology.
DSA pre-patterns for graphoepitaxy are typically generated in a hard mask layer (for example, silicon oxide) to prevent the solvent used in casting the self-assembly material (SA material) from dissolving the pre-pattern. The use of hard mask pre-patterns adds process complexity, as shown in the series of steps in FIG. 1A to 1D (Prior Art) using schematic layer diagrams. First, a hard mask layer 202 is deposited on a bottom layer 200 (e.g., silicon wafer or transfer layer), followed by a photoresist layer 204 (FIG. 1A). Patterning the photoresist layer in a lithographic process produces a photoresist pattern 206 (FIG. 1B). If the hard mask layer 202 does not also serve as an anti-reflective coating (ARC), a separate ARC layer can be applied to the substrate prior to the photoresist. The photoresist pattern 206 comprising trench area 208 is then transferred into the hard mask layer 202 by an additive or subtractive process, after which the photoresist is stripped, leaving a hard mask pre-pattern 210 (FIG. 1C) for self-assembly. Hard mask pre-pattern 210 includes all surfaces (e.g., surfaces 203, 205, and 207 in FIG. 1C) formed or uncovered by etching the hard mask layer and removing the photoresist pattern. If desired, the surface of the hard mask pre-pattern 210 can be further modified prior to applying a material for self-assembly. For example, a polymer can be chemically grafted onto the surface of the hard mask pre-pattern 210 to provide the appropriate surface properties for the hard mask pre-pattern 210 to guide the subsequent self-assembly process. In particular, such a modification of the hard mask pre-pattern 210 can be used to control the affinity of the pre-pattern for particular domains of the SA material. A SA material is then cast from a solvent onto the hard mask pre-pattern 210 to form layer 212, in this case allocating the SA material substantially in trench 208. The hard mask pre-pattern topographically directs the self-assembly of the SA material, to form ordered domains 214 and 216 in the trench areas 208 (FIG. 1D). The additional steps needed to generate a hard mask pre-pattern and modify its surface properties increase the cost of the DSA process.
In the chemical epitaxy technique, DSA chemical pre-patterns for chemical epitaxy are typically made from a two-layer stacked structure in a multi-step process. The two-layer stack consists of a photoresist layer disposed on a substrate. First, a topographic pattern is generated by imaging and developing the photoresist layer, uncovering a surface of the underlying substrate. Second, the uncovered surface of the substrate is damaged by exposure to plasma through the openings in the photoresist layer, causing a change in a surface property of the uncovered surface of the substrate. Third, the photoresist is removed to produce a chemically patterned surface for DSA consisting of damaged and undamaged surface regions of the substrate. Chemical epitaxy is also challenged by high cost and process complexity by introducing a functional underlying surface that can be damaged during the pattern transfer step.
Alternatively, a negative-tone photoresist that crosslinks and becomes less soluble in developer solvent in the exposed regions can be employed to create pre-patterns suitable for chemical epitaxy. For example, a thin layer of a crosslinking negative-tone photoresist (such as hydrogen silsesquioxane (HSQ)) is disposed on a substrate comprising a suitable surface for self-assembly. Then a chemically patterned surface is created by imaging the thin layer of crosslinking negative-tone photoresist and removing the non-crosslinked material (e.g., the photoresist in the non-exposed regions). In such a process, the thickness of the crosslinking negative-tone photoresist must be less than the thickness of the layer of the SA material in order to direct the self-assembly process by chemical rather than by topographical means.
Solvent non-compatibility and underlayer non-compatibility present major obstacles to integrating DSA directly into a standard lithographic process. The photoresist patterns formed after exposure and development of standard positive-tone photoresists are soluble in many organic solvents. High solubility of the patterned photoresist in the solvents used to apply the SA material, or interaction with the solvents, limits the utility of standard positive-tone photoresists in DSA (e.g., the solution of SA materials dissolves or collapses the photoresist pattern). A surface having controlled affinity for the various domains formed during self-assembly is necessary for controlling the orientation of the self-assembled structures; however, chemical reactions initiated by the imaging process and/or the subsequent bake and development processes have a high probability of altering the surface property of the underlayer and rendering the underlayer incompatible with the desired DSA morphology. For example, patterning a conventional positive-tone photoresist using a conventional aqueous tetramethylammonium hydroxide (TMAH) developer on top of an underlying surface can detrimentally impact the underlying surface properties responsible for the controlled affinity. Alternatively, a chemical “freezing” agent or surface curing agent can be used to render a non-crosslinked photoresist pattern insoluble in the solvent used for casting a SA material; however, these chemical treatments can also react with the underlayer to damage or change its surface properties. In addition, such chemical treatments constitute additional process steps, and can induce a dimensional change in the photoresist pattern. As mentioned previously, DSA will not work properly without the appropriate underlayer surface property.
A negative-tone photoresist that crosslinks and becomes less soluble in developer solvent in the exposed regions can be employed to produce photoresist patterns that will not dissolve in typical casting solvents used to apply SA materials. Unfortunately, negative-tone photoresists have historically offered limited resolution and suffered from poor profiles, microbridging, and/or swelling in organic solvents. Furthermore, some patterns can be difficult to image using a negative-tone mask. Finally, frozen positive-tone photoresist patterns and crosslinked negative-tone photoresist patterns are difficult to rework.
Therefore, new methods are needed for generating topographical or chemical pre-patterns for DSA that have fewer process steps, that have less risk of damaging the underlayer, and that preserve desirable underlayer surface properties.